1. Field of the Invention
The present invention relates to a wavelength division multiplexed optical processing device and an optical communication transmission path for transmitting wavelength division multiplexed optical signals while compensating or reducing at least one of dispersion slope of optical fiber transmission paths, wavelength dependency of optical amplifier gain, and accumulated optical noise generated by optical amplifiers.
2. Description of the Background Art
In recent years, much attentions have been attracted to the optical communication using wavelength division multiplexed optical signals in which optical signals in different wavelengths are multiplexed. For optical amplification of the wavelength division multiplexed optical signals, erbium-doped optical fibers formed by doping erbium in optical fibers are used, for example. An optical amplifier using erbium doped optical fiber is capable of amplifying the wavelength division multiplexed optical signals collectively so that it is well suited for the wavelength division multiplexed optical communication, and there is a report of a long distance transmission experiment which was conducted over a distance of 9000 km (see, H. Taga, et al., "110 Gbit/s (22.times.5 Gbit/s), 9500 km transmission experiment using 980 nm pump EDFA 1R repeater without forward error correction", Optical Amplifiers and Their Applications 1996, PDP5).
In a long distance wavelength division multiplexed optical signal transmission through optical fiber transmission paths using such optical amplifiers, the known factors that degrade the transmission characteristic includes accumulated wavelength dispersion for each signal wavelength caused by the wavelength dispersion slope of the optical fiber transmission paths, wavelength dependency of optical amplifier gains, and accumulate noise of optical amplifiers.
As for the accumulated wavelength dispersion, the transmission characteristic can be improved by the equalization realized by giving the same amount of wavelength dispersion as the accumulated wavelength dispersion but in an opposite sign at a receiving end. However, when a bit rate is high or when an absolute value of the accumulated amount is excessively large, it is difficult to improve the transmission characteristic by the equalization at a receiving end. There is also an easily conceivable method for compensating the dispersion slope itself by separating each wavelength component of the wavelength division multiplexed signals and cancelling the slope in a manner of equalization by giving appropriate dispersion to each wavelength component separately, but there has been no known scheme that can account for possible transmission degradation due to characteristics of wavelength separation elements.
As for the wavelength dependency of the optical amplifier gains, a flat optical amplifier gain can be realized over a wave wavelength range by inserting into the optical amplifier a gain equalizer element that has an inverse characteristic with respect to the gain wavelength dependency (see, P. F. Wysocki, et al., "Erblum-Doped Fiber Amplifier Flattened Beyond 40 nm Using Long-Period Grating", Optical Fiber Communication Conference (OFC) 1997, PD2). However, the practical realization of such a gain equalization element is associated with some technical difficulties.
As for the accumulated noise of optical amplifiers, it is the physical phenomenon that is unavoidable as long as optical amplifiers are used, so that high performance optical amplifiers with low noise factor can be employed in order to reduce the accumulated noise as much as possible (see, H. Taga, et al., "110 Gbit/s (22.times.5 Gbit/s), 9500 km transmission experiment using 980 nm pump EDFA 1R repeater without forward error correction", Optical Amplifiers and Their Applications 1996, PDP5). However, higher performance optical amplifiers are more expensive, and associated with a drawback that the reliability of light sources for optical pumping is low.